An optical waveguide device that uses an electro-optic crystal such as lithium niobate (LiNbO3) or lithium tantalate (LiTaO2), is formed by forming a metal film of titanium (Ti) or the like on a part of a crystal substrate, to be thermally defused, or to be patterned, after which it is proton exchanged or the like in benzoic acid, to form an optical waveguide, and thereafter an electrode is provided in the vicinity of the optical waveguide. As such an optical waveguide device that uses an electro-optic crystal, there is known for example an optical modulator as illustrated in FIG. 1.
In FIG. 1, an optical waveguide formed on a substrate 100 comprises; an input waveguide 101, a pair of branching waveguides 102 and 103, and an output waveguide 104. A signal electrode 105 and an earth electrode 106 are provided on the pair of branching waveguides 102 and 103, to form a co-planar electrode. In the case where a Z-cut substrate is used, in order to use the refractive index variation due to the electric field in the Z direction, the signal electrode 105 and the earth electrode 106 are arranged directly above the optical waveguides. More specifically, the electrodes are patterned with the signal electrode 105 on the branching waveguide 102, and the earth electrode 106 on the branching waveguide 103. Here in order to prevent the light that is propagated through the branching waveguides 102 and 103 from being absorbed by the signal electrode 105 and the earth electrode 106, a buffer layer (not illustrated in the figure) is provided between the substrate 100, and the signal electrode 105 and the earth electrode 106. For the buffer layer, an oxide silicon (SiO2) or the like of 0.2 to 2 μm thickness is used.
In the case where such an optical modulator is driven at high speed, the output end 105out of the signal electrode 105 is connected to the earth electrode 106 via a resistance (not illustrated in the figure) to make a travelling wave electrode, and a microwave electrical signal is applied from the input end 105IN of the signal electrode 105. At this time, due to the electric field generated between the signal electrode 105 and the earth electrode 106, the refractive indices of the branching waveguides 102 and 103 respectively change as +na and −nb, so that the phase difference of the light propagated on the branching waveguides 102 and 103 changes. Therefore a signal light that has been intensity modulated by Mach-Zehnder (MZ) interferometer, is output from the output waveguide 104. By changing the cross-section shape of the signal electrode 105 to control the effective refractive index of the microwave electric signal, and by matching propagation speeds of the light and the microwave electric signal with each other, high speed optical response characteristics can be obtained.
In the above optical modulator, one of the two branching waveguides is below the signal electrode while the other is below the earth electrode. Since the electrical field below the earth electrode is weaker than the electric field below the signal electrode, a difference occurs in the phase modulation amount between the two branching waveguides. Therefore, there is not complete push pull in the Mach-Zehnder interferometer, so that the wavelength of the signal light at the instant of switching the signal light from ON to OFF or from OFF to ON changes. That is to say, wavelength chirp occurs.
As a conventional technique for reducing this wavelength chirp, for example as illustrated in FIG. 2, a construction is proposed in which a polarization inversion region Pi in which the polarization direction of a crystal is inverted, is formed on a portion of a section (hereunder referred to as an interaction portion) INT in which light and electric field interact (refer for example to International Publication Pamphlet No. WO 04/053574). In this polarization inversion region Pi, the direction of change of the refractive index when an electric field is applied to the signal electrode 105 is opposite to that for the remaining regions Pn (non-polarization inversion regions) of the interaction portion INT. In the boundary portion between the polarization inversion region Pi and the non-polarization inversion region Pn, the positional relationship of the signal electrode 105 and the earth electrode 106 with respect to the pair of branching waveguides 102 and 103 is switched. Due to this configuration, the aforementioned difference in the phase modulation amounts that are respectively generated by the polarization inversion region Pi and the non-polarization inversion region Pn is cancelled out. Therefore it is possible to reduce the wavelength chirp of the modulation light.
Incidentally, due to the variety of recent optical modulation formats (for example multi-valued modulation format, optical polarization division multiplexing format, and the like), there are many cases where signals corresponding to a desired optical modulation format are generated, by combining a number of conventional optical modulators such as illustrated in FIG. 1 and FIG. 2. In this case, in order to reduce the size of the overall optical modulator, a plurality of optical modulators are integrated on a single chip. In the following description, individual optical modulator integrated on a single chip is referred to as “an optical modulation section”.
In the case where a plurality of optical modulation sections are arranged in parallel on a single chip, care is required so that cross talk does not occur between electrodes in the adjacent optical modulation sections. Furthermore, if the earth electrode disposed between the optical modulation sections is narrow, earthing becomes insufficient. Therefore it is necessary to have a sufficient spacing between adjacent optical modulation sections. In particular, in the case where the optical modulation sections include a polarization inversion region as illustrated before in FIG. 2, shifting the signal electrodes in the border portion between the polarization inversion region and the non-polarization inversion region, can make the spacing between the signal electrodes in the adjacent optical modulation sections narrow. Therefore it is necessary to further open up the spacing in the optical modulation sections.
More specially, as illustrated for example in FIG. 3, a configuration is assumed in which two optical modulation sections 100A and 100B are arranged in parallel on the same substrate, and the positions of the boundary portion between the polarization inversion region Pi and the non-polarization inversion region Pn are displaced for the respective optical modulation sections 100A and 100B, in the lengthwise direction of the substrate. In this configuration, a spacing S between the signal electrode 105A of the optical modulation section 100A and the signal electrode 105B of the optical modulation section 100B is narrowed to S′ in the border portion enclosed by the dotted line in the figure. Therefore, it is necessary to widen the spacing between a branching waveguide 103A of the optical modulation section 100A and a branching waveguide 102B of the optical modulation section 100B that is adjacent to the branching waveguide 103A, so as to obtain sufficient earthing in the abovementioned border portion. As is also clear from this specific example, there is a certain limit to miniaturization of the overall optical modulator by integrating a plurality of optical modulators on a single chip.
Furthermore, focusing on the configuration of an optical waveguide for applying input light to respective optical modulating sections arranged in parallel on a single chip, then as illustrated by the configuration example of FIG. 3, in the case where the single optical waveguide 111 is branched into two optical waveguides 113A and 113B by the optical branching section 112, and connected to the input waveguides 101A and 1018 of the respective optical modulation sections, the light output from a single light source (not illustrated in the figure) can be applied to the respective optical modulation sections 100A and 100B. In this case, it is possible to simplify the configuration of an optical transmission apparatus that includes a light source and an optical modulator.
However, in the configuration of the above optical modulator, if the spacing between the respective optical modulation section 100A and 100B becomes wide, it becomes necessary to make the radii of curvature of the curved waveguides 113A and 113B of approximate S shapes that connect between the optical branching section 112 and the input waveguides 101A and 1018 of the respective optical modulation sections, small. Therefore the increase in the bend loss (radiation loss) in the respective curved optical waveguides 113A and 1138 becomes a problem. If the radii of curvature are increased in order to suppress the increase in the bend loss, the length of the respective curved waveguides 113A and 113B (the distance between the branching section 112 and the respective optical modulation sections 100A and 100B) becomes long, so that the size in the length direction of the optical modulator becomes great. Normally, the overall length in the lengthwise direction of a single chip is restricted to the size of the wafer or the like from which the chip is cut. Therefore if the overall length of the chip cannot be extended to correspond to the extension amount of the respective curved waveguides 113A and 113B, the interaction portion INT of the respective optical modulation sections 100A and 100B must be shortened. If the interaction portion INT is shortened, the drive voltage increases, and hence a high output driver amplifier is necessary. As a result, the cost and power consumption of the optical modulator is increased.